Gasification of biomass or other solid fuel is a process whereby air or oxygen is limited in the gasification chamber to achieve thermal degradation of complex materials with only partial combustion of the fuel. This limited air process is referred to as starved air, substoichiometric air, or pyrolysis. The latter, scientifically defined as the thermal degradation of complex material in an inert atmosphere or a vacuum is used herein and conventionally understood in the art to mean starved air or substoichiometric air. The resultant gases from this thermal degradation or gasification are subsequently oxidized in a second unit operation utilizing staged oxidation or staged introduction of air to complete or nearly complete the oxidation, or burn off, process. In application, this gasification and oxidation of fuel often includes a third step in which thermal energy is recovered from the flue gas using a heat recovery device such as a steam boiler or air-to-air heat exchanger. This thermal energy can be used, for example, to generate process steam, electrical power, or as a heat source for commercial applications such as supplying clean hot air to a lumber drying kiln.
It is important to be able to hold air within a gasifier at low (20–40 percent) substoichiometric air percentages. Maintaining low substoichiometric air percentages, specifically the percentage of air required for complete combustion, is critical because this allows the gasifier to maintain temperatures below the melting points of many solids and salts that start to sublimate, vaporize, and/or combust when the temperature gets above approximately 950 degrees F. At temperatures between 1100 and 1300 degrees F. most solids sublimate and go out the stack.
Conventional gasification systems, due to numerous sources of air leakage and/or poor air control, operate at substoichiometric air percentages of 40 to 60 percent. Thus these machines operate at higher temperatures. Operation at higher temperatures is undesirable since such operation leads to loss of ash, increased particulate emissions, and residual solid, and also leads to formation of corrosive slag. Slag is formed when salts are melted within the gasification chamber. Formation of corrosive slag attacks metal components within the gasifier and in downstream equipment, including grates and boiler tubes. When vaporized solids are discharged, a potentially valuable by-product of the process is lost since the ash or residual solids can have value. For example, when poultry litter is gasified, the residual ash is useful as fertilizer feedstock. In precious metal recovery applications, particulate carryover allows gold, silver, platinum, etc. to be discharged to the atmosphere. When coal is gasified, it is important to retain the residual sulfates to prevent acid-producing sulfur from being discharged to the atmosphere. Other conventional designs which attempt to operate at lower temperatures are not able to control the air at such low percentages, resulting in considerable hot-spotting and clinker formation wherever tramp air enters the system.
Fuel pile configuration within a gasifier is very important to achieving uniform gasification. A fuel pile which is peaked in the center causes uneven burning of the pile. A concave fuel pile causes build up and non-gasified fuel at the perimeter of the gasifier. The optimum fuel pile shape is an elliptical to relatively flat contour, and is achieved by careful synchronization of fuel feed with ash removal. Uniform gasification is further promoted when the dome contour mirrors fuel pile shape. However, conventional gasifiers use a circular dome.
Because of the finished size and weight of conventional gasification and oxidizing incinerators, they are constructed on site in a process which involves assembling an outer shell, including dome, sidewalls, furnace bed with grates and other assemblies, and fuel feed mechanism, and lining the unit with refractory brick or “gunning” refractory material on the interior sidewalls of the unit, and so on. This is a costly, labor intensive process. These gasification units tend to have many sources of air leakage, or “tramp air”, as a result of this on-site construction. Tramp air is also associated with the use of feed, grate, and ash removal assemblies, especially in cases where these assemblies are moveable.
Conventional gasification systems, incorporating metal components, can operate with some success when burning uniform fuels such as like-sized wood chips. However, these units are not successful in burning non uniform solid fuels. Biomass fuels such as agricultural waste, bitumen, bovine, swine, and poultry manures, poultry carcasses, et cetera, are non-uniform in size, shape, water content, and material. Examples of other solid fuels which are non uniform include coal tailings, municipal solid waste, industrial waste, and medical waste. During combustion, non uniform fuels have a tendency to have areas of locally high temperatures, or hot spots. Hot spotting causes warping and failure of metal components, even when these components are provided with cooling mechanisms. Further, competitive metal units deteriorate rapidly if the system is cycled, for example, during start up and shut down. Use of specialized high-temperature metals to compensate for these problems is costly, and not always successful.
Conventional air injection systems, used to aerate the fuel pile within the gasifier, use plenums within the furnace bed and walls where the tuyeres are holes formed in the plenum. Plenums are large enclosed spaces formed below the furnace bed surface. Because of the location of the plenum beneath the fuel pile, the tuyeres tend to clog with ash. Further, changes in air pressure within a conventional gasifier cause ash to be drawn back into the plenum to the point of filling the plenum with ash. Because the plenum is within the furnace bed, maintenance of the conventional air injection systems is difficult and costly. Because the location of air injection is associated with locally higher burn rates and thus locally high temperatures, use of metal as a fabrication material is problematic. Even when provided with cooling mechanisms, metal air injection systems are associated with clinker formation and tend to fail, for example during power failure or when the fuel pile burns down.
Following initial, partial primary combustion within a gasifier, combustion gases are oxidized within a secondary combustion unit. Oxidation is intended to burn off remaining combustibles such as CO, hydrocarbons, and VOCs. Some environmental codes require that oxidation temperatures reach 2200 degrees F. to insure complete burnoff of these compounds. Unfortunately, depending upon the nitrogen content of the fuel and other variables, NOx may begin to form at 1800 degrees F. and increases exponentially with increasing temperature. For purposes of this discussion, the critical NOx formation temperature used is 2200 degrees F.
Modern oxidizers are required to balance complete burnoff of CO, hydrocarbons, and VOCs while minimizing formation of NOx, where the term “complete” is understood to mean “essentially complete” or “nearly complete” with only trace levels of the compound present in the flue gas, the trace levels being far below those levels allowed by code. Non-staging oxidizers reach flame temperatures above 2200 degrees F., form NOx, and are then cooled, thus burning off much of the CO, hydrocarbons, and VOCs but producing flue gas having high levels of NOx. Staged oxidizers exist in the prior art which are intended to achieve a better balance. Lewandowski et al. discuss a two stage oxidizer in U.S. Pat. No. 5,707,956 for reduction of NOx emissions from waste gas, where water and natural gas are injected into the waste gas to control combustion temperatures, and where there is no physical barrier separating the stages. U.S. Pat. No. 4,285,193 to Shaw et al describes a two zone combustion chamber where catalysts are used to provide efficient combustion. Both these patents require the use of external additives to achieve efficient combustion.
Heat energy recovery from the clean flue gas discharged from the oxidizer can be accomplished using many devices such as turbine systems, boilers, heat exchangers, and external combustion engines. Use of turbine systems in combination with heat exchanger systems are known, but are associated with high equipment costs and require input temperatures significantly lower than that produced by the gasifier/oxidizer system, to prevent damage to metal components. Use of external combustion engines such as Stirling cycle engines having gasification units as their heat energy source have also been previously contemplated. External combustion engines are reliable producers of electrical power at a fraction of the cost of turbine systems. However, firing external combustion engines using gases which are products of combustion has been unsuccessful in practice because of the high levels of particulate found in the flue gas of conventional gasifiers, and because of the low operating temperatures of conventional gasifiers.
There is a need for a pyrolyzing gasifier which can operate at low substoichiometric air percentages (10–30 percent) to maintain internal gasifier temperatures below 1100 degrees F. There is a need for a pyrolyzing gasifier which can tolerate general high temperatures, as well as hot-spotting and clinker formation. There is a need for an oxidizer which can provide complete burnoff of CO, hydrocarbons, and VOCs while forming minimal NOx, and still maintain exit temperatures at code levels. There is a need for a staged oxidizer which can achieve efficient combustion without requiring the use of external additives. There is a need for a reliable and low-cost system for pyrolyzing non uniform solid fuels such as biomass and wastes as a means of energy production. There is a need for a method of pyrolyzing non uniform solid fuels such as biomass, waste coal and bitumen as a means of reducing the volume of unwanted waste material which must be landfilled or otherwise stored. There is a need for a method of pyrolyzing non uniform solid fuels which transforms the fuel into recoverable, useful heat energy and useful ash.